Lubricating compositions are compositions applied between the surfaces, in particular metallic surfaces, of moving parts. They make it possible to reduce the friction and wear between two parts that are in contact and moving with respect to each other. They also serve to dissipate part of the heat energy generated by this friction. The lubricating compositions form a protective film between the surfaces of the parts on which they are applied.
The compositions used for the lubrication of mechanical parts are generally constituted by a base oil and additives. The viscosity of the base oil, in particular of petroleum or synthetic origin, varies when the temperature is changed.
Indeed, when the temperature of a base oil increases, its viscosity reduces, and when the temperature of the base oil reduces, its viscosity increases. Now, the thickness of the protective film is proportional to the viscosity and therefore also depends on the temperature. A composition has good lubricating properties if the thickness of the protective film remains substantially constant whatever the conditions and duration of use of the lubricant.
In an internal combustion engine, a lubricating composition can be subjected to changes in external or internal temperature. The changes in external temperature are due to the variations in temperature of the ambient air, such as the variations in temperature between summer and winter for example. The internal changes in temperature result from the running of the engine. The temperature of an engine is lower during its start-up phase, in particular in cold weather, than during prolonged use. As a result, the thickness of the protective film can vary in these different situations. A need therefore exists to have available a lubricating composition having good lubrication properties and the viscosity of which is not significantly affected by variations in temperature.
It is known to add additives that improve the viscosity of a lubricating composition. These additives have the function of modifying the rheological behaviour of the lubricating composition. They make it possible to promote a substantially constant viscosity over a temperature range at which the lubricating composition is used. For example, these additives limit the reduction in the viscosity of the lubricating composition when the temperature increases or limit the increase in the viscosity of the lubricating composition when the temperature reduces.
The additives improving the viscosity (or additives improving the viscosity index) currently used are polymers such as the polyalpha-olefins, methyl polymethacrylates, copolymers resulting from the polymerization of an ethylene monomer and an alpha-olefin. These are high molecular weight polymers. In general, the contribution of these polymers to controlling the viscosity is greater, the higher their molecular weight. However, the high molecular weight polymers have the drawback of having a low permanent shear strength compared with polymers of the same nature but a smaller size.
Now, a lubricating composition is subjected to significant shear stresses in particular in internal combustion engines, where the surfaces subject to friction have a very small clearance and the pressures exerted on the parts are high. These shearing constraints on the high molecular weight polymers lead to macromolecular chain cleavages. The polymer thus degraded no longer has thickening properties, and the viscosity drops irreversibly. This loss of permanent shear strength therefore leads to a degradation of the lubrication properties of the lubricating composition.
The polymers of the prior art, in particular PMMA (methyl polymethacrylates) have a shear thickening behaviour. At a high shear rate, the PMMA chain breaks. This results in the formation of two molecules having approximately half of the molar weight of the initial PMMA. The total hydrodynamic volume of these two small molecules is less than that of the initial PPMA, which leads to a smaller contribution to the viscosity and this results in a reduction in the viscosity.
The ethylene-alphaolefin copolymers having a high ethylene content are additives improving the viscosity and are stable under shear. However, these polymers have the drawback of aggregating in the compositions containing them and lead to lubricating compositions that are extremely viscous, such as gels. This aggregation generally takes place under ambient conditions or during cooling.
Therefore the Applicant has set himself the objective of the formulation of novel lubricating compositions the viscosity of which is better controlled compared with respect to the lubricating compositions of the prior art. In particular, his objective is to provide novel rheological additives, the behaviour of which when they are introduced into a base oil, is opposite as regards temperature change compared with the behaviour of the base oil and the rheological additives of polymer type of the prior art.
This objective is achieved thanks to novel rheological additives capable of associating, in order to optionally form a gel, and exchanging in thermoreversible manner. Unlike the base oil which liquefies when the temperature increases, the additives of the present invention have the advantage of thickening the medium in which they are dispersed when the temperature increases. This characteristic results from the associated use of two particular compounds, a copolymer bearing diol functions and a compound comprising boronic ester functions.
Polymers, of which at least one monomer comprises boronic ester functions are known from document WO2013147795. These polymers are used for the production of electronic devices, in particular for devices in which it is desired to obtain a flexible user interface. These polymers are also used as synthesis intermediates. They allow the functionalization of the polymers by coupling with luminescent groups, electron-transporter groups, etc. The coupling of these groups is carried out by standard organic chemistry reactions, involving the boron atom, such as for example Suzuki coupling. However, no other use of these polymers in the field of lubricating compositions, nor an association with other compounds is envisaged.
A copolymer resulting from the copolymerization of a methyl methacrylate (MMA) monomer and a glyceryl methacrylate monomer optionally protected by a boronic ester (namely butyl boronic acid adduct of glyceryl methacrylate (BBA-GMA)) is known from document U.S. Pat. No. 4,401,797. This copolymer forms a hydrogel in the presence of water and is used for the production of contact lenses. However, no other use of this copolymer in the field of lubricating compositions, nor a association via exchangeable chemical bonds with other compounds is envisaged.
Document EP0570073 discloses an additive which improves the viscosity index of a lubricating composition in which it is added. This additive is a copolymer resulting from the polymerization of 1-(methacryloylethoxy)-4,4,6-trimethyl-dioxaborinane and a methacrylate of a linear (C12-C18) alkyl. This additive belongs to the family of the borate compounds which can be represented by the general formula B(OR)3 with R an alkyl or aryl group. This additive does not belong to the family of the boronate compounds which can be represented by the general formula R—B(OR)2 with R an alkyl or aryl group. This additive cannot be associated with other compounds via exchangeable chemical bonds.
Unexpectedly, the Applicant observed that at low temperature, the polydiol copolymer of the invention is not or only slightly cross-linked by the compounds comprising boronic ester functions. When the temperature increases, the diol functions of the copolymer react with the boronic ester functions of the compound containing them by a transesterification reaction. The polydiol statistical copolymers and the compounds comprising boronic ester functions then link together and can exchange. Depending on the functionality of the polydiols and of the compounds comprising boronic ester functions, as well as depending on the composition of the mixtures, a gel may form in the base oil. When the temperature reduces again, the boronic ester bonds between the polydiol statistical copolymers and the compounds containing them break; the composition loses its gelled character, if applicable.
The Applicant has set himself the objective of the formulation of novel rheology additives which are more stable under shearing compared to the compounds of the prior art. This objective is achieved thanks to novel rheological additives which can associate and cross-link in a thermoreversible manner. Unlike the polymers of the prior art, it was noted that the molar weight of the copolymers of the invention is not or only slightly modified when a high shear rate is applied. The copolymers of the invention therefore have the advantage of being more stable under shearing stresses.